Energy Conversion and Management 48 (2007) 2556–2564
www.elsevier.com/locate/enconman
Investigation and improvement of ejector refrigeration system
using computational fluid dynamics technique
K. Pianthong
a,*
, W. Seehanam a, M. Behnia b, T. Sriveerakul a, S. Aphornratana
c
a
c
Department of Mechanical Engineering, Ubon Ratchathani University, Ubon Ratchathani 34190, Thailand
b
Postgraduate Study, University of Sydney, Sydney, NSW, Australia
Department of Mechanical Engineering, Sirindhorn International Institute of Technology, Thammasart University, Pratumthani 12121, Thailand
Received 4 August 2006; accepted 29 March 2007
Available online 29 May 2007
Abstract
Ejector refrigeration systems are usually designed to utilize low grade energy for driving the cycle. They also have low maintenance
cost because they operate without a compressor. Mainly, the ejector performance directly affects the refrigerating performance. Therefore, an investigation on the characteristics and an efficient design of the ejector are important to improve ejector refrigeration systems.
In this study, the computational fluid dynamics (CFD) code, FLUENT, is employed to predict the flow phenomena and performance of
CPM and CMA steam ejectors.
The ejector refrigeration system, using water as the working fluid, is operated at 120–140 °C boiler temperature and 5–15 °C evaporator temperature. CFD can predict ejector performance very well and reveal the effect of operating conditions on an effective area that is
directly related to its performance. Besides, it is found that the flow pattern does not depend much on the suction zone because the results
of axisymmetric and 3D simulation are similar. This investigation aids the understanding of ejector characteristics and provides information for designing the ejector to suit the optimum condition.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Ejector; Ejector refrigeration; Computational fluid dynamics (CFD)
1. Introduction
The ejector refrigeration system was firstly developed by
Maurice Leblanc in 1910 [1]. This refrigeration system utilized low grade thermal energy or waste heat instead of
using electricity. The main advantage of this system is its
having fewer moving parts (no compressor). It is, therefore,
very low in wear and significantly durable. It is also suitable to operate using water as a refrigerant. However, it
usually has a very low coefficient of performance (COP),
and this becomes the critical issue and disadvantage of this
system.
Fig. 1 shows the operating cycle of the ejector refrigeration system. Comparing to the typical refrigeration cycle or
*
Corresponding author. Tel.: +66 45 353 382; fax: +66 45 353 333.
E-mail address: K_Pianthong@yahoo.com (K. Pianthong).
0196-8904/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.enconman.2007.03.021
vapor compression cycle, it can be seen that the ejector, the
boiler (or steam generator) and the circulating pump are
used to replace the compressor. The high pressure refrigerant, boiled in the boiler, is the primary gas feeding to the
primary nozzle. It then expands through the nozzle throat
at supersonic speed and causes a low pressure area where it
connects to the evaporator. Therefore, the refrigerant in
the evaporator can boil and evaporate easily. The heat
absorbed at the evaporator is the refrigerating capacity.
The evaporated refrigerant is called the secondary gas.
The primary and secondary gases are mixed and flow
through the ejector to the condenser. The liquid refrigerant
is pumped back to the boiler partly, and some portion is
fed through the expansion valve and evaporator to complete the cycle. It can be seen that the refrigeration performance of the system depends much on the performance of
the ejector to induce the refrigerant flow rate through the
evaporator.
K. Pianthong et al. / Energy Conversion and Management 48 (2007) 2556–2564
Fig. 1. Operating cycle of ejector refrigeration cycle.
Usually, the two parameters indicating ejector performance are entrainment ratio (Em) and critical back pressure (CBP). Em is defined as shown in Eq. (1), while
CBP is the final pressure (condensing pressure) with the
ejector working at its maximum capability.
Entrainment ratio Em ¼
mass flow rate of secondary flow
mass flow rate of primary flow
ð1Þ
Typically, ejectors are categorized in two types based on
the mixing concept at the primary nozzle exit. The first one
is the constant mixing area (CMA) ejector in which the exit
of the primary nozzle is placed at the constant area throat.
The second type is the constant pressure mixing (CPM)
ejector in which the exit of the primary nozzle is placed
at the converging area throat. The setup of both the
CMA and CPM ejector are shown in Fig. 2. These two
ejectors are suitable to use in different situations. The
CMA is capable of drawing more mass flow rate than the
CPM, but the CPM is more flexible or suitable to operate
in wider condensing pressure ranges.
In the past, the performances of the two ejectors have
already been researched in ejector refrigeration systems.
However, not much information on the ejector characteristics was reported in detail when there were many parameters involved. Also, they were mainly experimental works
and were quite limited in the testing conditions. Therefore,
this study aims to use the computational fluid dynamics
(CFD) technique to simulate ejector performance in various conditions and to suggest the best possible solutions.
Fig. 2. Two typical ejector types: (a) constant pressure mixing ejector and
(b) constant mixing area ejector.
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Recently, a new concept of ejector design has been proposed by Eames [2]. It is called the constant rate of momentum change (CRMC) concept for which the ejector is
expected to combine the benefits of CMA and CPM ejectors and perform better. In the CRMC concept, a new profile of the diffuser or diverging section of the ejector is
proposed. It is claimed that the entrainment ratio of the
CRMC is slightly higher than that of the CPM, and the
CBP is significantly improved. Garris et al. [3] try to
enhance the efficiency of the ejector by reducing the speed
of the primary gas by allowing it to expand through a self
rotating skew. By rotating the primary gas, the loss during
mixing will be lower. From the Garris idea, Chang and
Chen [4,5] have developed the petal nozzle and found that
the Em and CBP of the ejector can be higher. However,
Garris’s and Chang’s ejectors are quite complicated in
structure compared to a typical ejector and are difficult
to use in practice.
For many years, researchers have tried to investigate
and describe the phenomena of ejector flow in order to
develop a high performance ejector. Keenan and his team
[6,7] were the first group who proposed an ejector theory.
It is a one dimensional ejector flow theory and widely used
to predict the properties of the fluid along the ejector axis
based on mixing and gas dynamics theory. This theory
was widely used, however, it could not describe the constant capacity effect when the exit pressure is decreased.
Later, Munday and Bagster [8] successfully adopted the
effective area concept (as shown in Fig. 3) within the calculation and showed good agreement with experiment. They
studied this phenomenon and described that the primary
and secondary fluids do not mix until their flow velocities
reach the sonic condition. It is similar to the flow passing
through the throat of the convergent–divergent nozzle
and choking (of secondary fluid) phenomenon occurs.
The flow area where the secondary fluid chokes is called
‘‘effective area’’ (as shown in Fig. 3).
Then, the ejector performance in the refrigeration system can be predicted. However, these concepts are not
proved yet due to the limitation of experimentation and
measurements, and there are still many factors involving
the ejector performance.
Riffat et al. [9] simulated the flow behavior inside the
ejector of the refrigeration system using methanol as refrigerant. He found that the CFD results agreed well with the
experimental results and can be used to predict other various conditions. Rusly et al. [10,11] investigated the flow
Fig. 3. Effective area occurring in the ejector throat.
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characteristics of the ejector in a refrigeration system by
using the real gas model in the commercial code, FLUENT, and found good agreement as well.
In this paper, the CFD code, FLUENT, is employed to
investigate the flow phenomena and performance of two
typical ejectors used in refrigeration systems, which are
the CMA and CPM ejector. The results are validated with
experiments and simulation in other various conditions.
Thus, the most preferable conditions can be applied in system design.
2. CFD for flow simulations
2.1. Computational modeling for ejector flow
The ejector model used in this study is shown in Fig. 4.
The model is composed of the primary nozzle and the ejector. The primary nozzle accelerates the high speed gas and
induces the secondary flow through the ejector. The ejector
consists of four parts, which are the secondary inlet, mixing
chamber, throat, and diffuser. The primary nozzle is usually
placed at the entrance of the secondary inlet. However, the
position of the primary nozzle, called nozzle exit position or
NXP, can be varied and affects the ejector performance.
The CFD code used in this study is FLUENT (version
6.0.12). The model ejector is the one used in Chunnanond’s study [12]. The ejector geometry is set as axisymmetric. About 48 000 nodes of quadrilateral mesh are
used. The dense meshes are preset at the mixing zone
along the exit of the primary nozzle as shown in Fig. 4.
This is to cope with the high gradient properties around
that area. The solving method is couple implicit. The realizable k e turbulence model is selected while the standard near wall function is used in the near wall
treatment. Boundary conditions are the pressure inlet
and outlet. The energy equation is included, while the
fluid property is defined as an ideal gas. In addition, a
three dimensional model (3D) is also investigated. This
is to determine the effect of the third dimension, compared to the axisymmetric model (ASXM), on the ejector
performance at the area around the suction pipe. The
hexahedral cell, 5,000,000 nodes, is used in the 3D model
as shown in Fig. 5.
2.2. Comparison of ASXM and 3D results
The 3D model of CPM ejector was simulated in order to
check whether the suction pipe has any effects on the
Fig. 4. Ejector geometry (2D) used in the CFD simulation.
Fig. 5. 3D ejector geometry used in the CFD simulation.
K. Pianthong et al. / Energy Conversion and Management 48 (2007) 2556–2564
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Fig. 6. Comparison of wall static pressure along the ejector from 2D (AXSM) and 3D model.
Fig. 7. Validation of the CFD and experimental results (a) at various boiler temperatures and (b) at various evaporator temperatures.
entrainment ratio of the ejector. The results of ASXM and
3D simulations are compared in Fig. 6. It shows very close
values of static pressure along the ejector axis. Other properties are also very close. In both models, the pressures
gradually increase and slightly fluctuate along the mixing
chamber and ejector throat and then smoothly increase in
the diffuser. From this comparison, it maybe summarized
that the ASXM is good enough to give accurate results,
and the 3D model is not necessary for further investigation.
2.3. Validation of CFD simulations
Validation of the CFD results using experimental work
has been done in this study and also confirmed with the
work of Chunnanond [13]. The effect of the condensing
pressure at various boiler temperatures and evaporator
temperatures has been investigated. For example, the
results are shown in Fig. 7. The results of Em and CBP
from CFD are slightly different from the results from the
experiments, around 5%.
3. Results
It is well known that the ejector is the key equipment in
the ejector refrigeration cycle because it determines the
mass flow rate of the refrigerant in the evaporator (i.e.
refrigerating capacity) and also the condensing pressure
(i.e. heat rejecting capacity). This study, therefore, investigates the effects of various operating conditions and ejector
geometries on the Em and CPB of the CPM and CMA
ejectors.
3.1. Effect of operating conditions
The change of operating condition certainly affects Em
and CBP. Detailed investigations are performed by CFD
here. Fig. 8, shows that a higher boiler temperature gives
a higher CBP but lower Em in both CPM and CMA ejectors. The evaporator temperature also affects the ejector
performance by increasing Em and CBP when the evaporator temperature increases in both ejector types. However, at the same operating conditions, the CMA usually
gives a higher mass flow rate or Em, but yields a lower
CBP.
3.2. Effect of ejector geometry
3.2.1. Effect of NXP on ejector performance
In this investigation, the ejector performance is determined when the nozzle exit position (NXP) is varied at various operating conditions. The results are shown in Fig. 9.
It is found that a higher entrainment ratio can be obtained
when the NXP is moved further from the ejector inlet
(negative direction). By doing this, the effective area in
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Fig. 8. Effect of operating conditions on ejector performance (a) at various boiler temperature and (b) at various evaporating temperature.
Fig. 9. Effect of NXP on ejector performances (a) at various boiler temperature and (b) at various evaporating temperature.
the ejector throat is getting bigger, and therefore, Em is
higher. However, there is only one optimum position. If
the NXP is moved too far, the momentum of the primary
gas will be lower and cause a lower Em. Therefore, the
CFD simulation is very useful in this case to decide the
most suitable NXP in the actual system in particular operating conditions.
3.2.2. Effect of throat length on ejector performance
In this study, the throat length (TL) is the variable, while
NXP = 0 and the mixing chamber is 125 mm (see Fig. 4).
Both the CPM and CMA ejectors are investigated at various operating conditions as before.
Fig. 10 shows the effect of the throat length on the ejector performance at various operating conditions. It reveals
Fig. 10. Effect of throat length on ejector performances (a) CMA ejector and (b) CPM ejector.
K. Pianthong et al. / Energy Conversion and Management 48 (2007) 2556–2564
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Fig. 11. Effect of throat length on entrainment ratio of an ejector (a) at various boiler temperature and (b) at various evaporator temperature.
that the longer throat ejector gives a higher CBP, but the
Em is constant. In Fig. 10a, for the CMA ejector, the optimum TL should be between 95 and 120 cm. In Fig. 10b, for
the CPM ejector, the optimum TL is 130 cm. If the TL is
too long, the CBP of the ejector will decrease. However,
the most suitable throat length also depends on the shape,
other dimensions of the ejector and operating conditions.
Fig. 11 shows the effect of the TL on the maximum Em
at various operating conditions. They confirm that increasing TL does not affect Em. It only affects CBP.
From the above investigations, it can be summarized
that the operating conditions and ejector shape or geometry directly affect the ejector performances. In practice, it
is difficult to design one ejector to perform well at all conditions and experiment with many ejector geometries. CFD
simulation, therefore, is very useful to provide basic understandings of the involved parameters. It also helps to determine the most suitable setup before building the real ejector
refrigeration system. Furthermore, CFD simulation can
reveal the flow phenomena inside the ejector relating to
its performance very well as shown in the next section.
4. Discussions
4.1. Flow phenomena inside an ejector
One benefit of CFD investigation is the numerical visualization. The flow phenomena inside the ejector can be
depicted from the post processing and used to support
the quantitative results.
Fig. 12 describes the Mach number plot of the flow in
the 3D ejector model. The operating conditions are
130 °C boiler temperature, 10 °C evaporator temperature
and 40 mbar condensing pressure. From the plot, the flow
near the nozzle exit along the center line of the ejector is
very high and fluctuates because of the expansion shock
waves. The secondary flow velocity at the ejector entrance
is very low. However, after it mixes with the primary flow,
it gains momentum, and they accelerate together. Then, the
flow velocity reduces at the diffuser. At the mixing cham-
ber, the velocity difference of the fluid at the ejector wall
and at the primary fluid core is very high. This causes a separate flow layer. The high speed primary flow acts as
another wall, so the choking condition of the secondary
flow can occur. It can also be noticed that the flow velocity
at the suction tube is very low compared to that in the mixing chamber or in the throat. This reduces the effect of the
suction tube shape on Em. That is why the ASXM and 3D
cases give significantly close results.
4.2. Reverse flow phenomena
In practice, only the on design operating conditions are
of interest in the ejector flow. The on design operating condition means the condition that the ejector can still perform
at its constant Em while the condensing pressure is
decreased. However, understanding the off design phenomena is also helpful. In this study, it is found that there is
always a reverse flow when the condensing pressure is
increased higher than the critical or choking point of the
ejector. In the CFD results, at reverse flow conditions,
the secondary flow in the ejector throat re-circulates or
even cycles. This makes the flow obstruct and reduce Em
eventually as shown in Fig. 13.
Normally, the reverse flow occurs at the diffuser, not in
the ejector. The flow velocity is decreased, and its kinetic
energy or velocity pressure is converted to static pressure.
The reverse flow at the throat or mixing chamber is undesirable and should be avoided. However, in the experiments, it is very difficult to detect or visualize the
position of the reverse flow phenomena. Therefore, the
CFD visualization is very useful in this case.
4.3. Effect of an effective area on ejector performance
The effective area concept, previously shown in Fig. 3, is
clearly described by the CFD visualization. It is a very
important phenomenon to indicate the Em capacity. Figs.
14–16 show that the effective area is dependent on the operation conditions. The core jet of the primary flow acts as
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Fig. 12. Mach number profile in the 3D case.
Fig. 13. Path lines in the reverse flow phenomena.
Fig. 14. Sizes of the jet cores at different boiler temperature. (a) 120 °C (b) 130 °C and (c) 140 °C.
Fig. 15. Sizes of the jet cores at different evaporator temperature (a) 5 °C, (b) 10 °C and (c) 15 °C.
K. Pianthong et al. / Energy Conversion and Management 48 (2007) 2556–2564
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Fig. 16. Sizes of the jet cores at different condenser pressure (a) 20 mbar, (b) 25 mbar, (c) 30 mbar, (d) 35 mbar (CBP), (e) 38 mbar and (f) 40 mbar.
another wall. Therefore, the case that has the bigger effective area has the better Em (120 °C boiler temperature in
this case). Fig. 15 shows the effective area when the evaporating temperature is varied. At higher evaporating temperature, the effective area is bigger, thereby giving the higher
Em. These results correspond well with the performance
investigation in the previous sections.
The ejector back pressure or condensing pressure also
affects the ejector performance. If the shape of the jet core
is examined carefully, while the condensing pressure is
lower than the critical back pressure (CBP), the jet core size
(i.e. effective area) does not change. Therefore, Em is constant. However, when the condensing pressure is higher
than the critical back pressure (CBP), the jet core is smaller
(i.e. bigger effective area), but Em does not increase. This
can be summarized that the effective area concept is proved
only at the on design operating conditions.
eration system. The CFD results have been validated with
experimental results. The axisymmetric and 3D cases have
been compared in order to determine if the shape of the
suction tube has affected the ejector performance. The
results show very similar solutions because the flow velocity at the suction is very low and is not significant to the
overall flow behavior. The effects of various operation conditions on the ejector performance have been investigated.
Ejector shapes or geometries are also varied and the ejector
performance simulated. The CFD visualization becomes a
great benefit in the study because it can reveal the phenomena inside the ejector in detail. In summary, the overall
view points on ejector performance related to its flow phenomena can be understood and become very useful tools to
design an appropriate ejector for each particular case.
4.4. Effect of mixing process on build up of static pressure
This research is financially supported by the Thai Research Fund (TRF), Contract No. MRG4680175.
In the mixing process of the primary and secondary
fluid, the momentum of the two fluids is exchanged
through the flow layer. The efficiency of the mixing can
directly affect the regained pressure (build up of static pressure) of the ejector. The two factors involved are the size of
the mixing chamber and the mixing period. If the size of the
mixing chamber is small, the momentum transfer is quite
complete. Therefore, the ejector can give high static pressure such as the CPM ejector. In addition, when the mixing
chamber is small, the effective area will be small as well,
and the ejector will give low Em. For the mixing period,
if there is a long mixing period, the momentum transfer will
be quite complete. That is why the reasonably long throat
ejector can give higher static pressure or work well at
higher CBP.
5. Concluding remarks
This study employs CFD techniques to investigate the
flow characteristics of the ejector used in an ejector refrig-
Acknowledgement
References
[1] Chunnanond K, Aphornratana S. Ejectors: application in refrigeration technology. Renew Sust Eng Rev 2004;8:129–55.
[2] Eames IW. A new prescription for design of supersonic jet pumps:
constant rate of momentum change method. Appl Therm Eng
2002;22:121–31.
[3] Hong WJ, Alhussan K, Zhang H, Garris Jr CA. A novel thermally
driven rotor – vane/pressure-exchange ejector refrigeration system
with environmental benefits and energy efficiency. Energy 2004;29:
2331–45.
[4] Chang YJ, Chen YM. Enhancement of a steam-jet refrigerator using
a novel application of the petal nozzle. Exp Therm Fluid Sci
2000;22:203–11.
[5] Chang YJ, Chen YM. Enhancement of a steam-jet refrigerator using
a novel application of the petal nozzle. J Chin Inst Eng 2000;23:
677–86.
[6] Keenan JH, Neumann EP. A simple air ejector. J Appl Mech-T
ASME 1942;64:75–81.
[7] Keenan JH, Neumann EP, Lustwerk F. An investigation of ejector
design by analysis and experiment. J Appl Mech-T ASME 1950;72:
299–309.
2564
K. Pianthong et al. / Energy Conversion and Management 48 (2007) 2556–2564
[8] Munday JT, Bagster DF. A new theory applied to steam jet
refrigeration. Ind Eng Chem Proc DD 1997;16(4):442–9.
[9] Riffat SB, Omer SA. CFD modelling and experimental investigation
of an ejector refrigeration system using methanol as the working fluid.
Int J Eng Res 2001;25:115–28.
[10] Rusly E, Aye L, Charters WWS, Ooi A, Pianthong K. Ejector CFD
modelling with real gas model. In: Proceedings of the 16th annual
conference of mechanical engineering network Thailand; 2002. p. 150–5.
[11] Rusly E, Aye L, Charters WWS, Ooi A. CFD analysis of ejector in a
combined ejector cooling system. Int J Refrig 2005;28:1092–101.
[12] Chunnanond K, Aphornratana S. An experimental investigation of a
stream ejector refrigerator: the analysis of the pressure profile along
the ejector. Appl Therm Eng 2004;24:311–22.
[13] Chunnanond K. A study of steam ejector refrigeration cycle,
parameters affecting performance of ejector. Ph.D. Thesis of Sirindhorn International Institute of Technology University 1994.